A linear encoder is known as a device that is attached to a manufacturing machine or a measuring machine to detect a position of a linearly movable unit. There are a variety of linear encoders such as an optical linear encoder, a capacitive linear encoder and an electromagnetic induction linear encoder, which employ different detection methods. In particular, as typical electromagnetic induction linear encoders, for instance, devices disclosed in Patent Literature 1 (JP-A-2009-276306) and Patent Literature 2 (JP-A-2011-247600) are known.
The electromagnetic induction linear encoder includes an elongated encoder scale having an induction electrode pattern and an encoder head slidable along the encoder scale. An electric current induced into the induction electrode by a sliding movement of the scale relative to the head is detected by a pick-up coil of the encoder head, and a displacement of the scale is detected by, for instance, counting the induction electrode(s) having passed through.
For the electromagnetic induction linear encoder, an electric conductor of the induction electrode should preferably be made of a material with a low electrical resistance to increase an electric current induced into the electric conductor. Accordingly, a metal with a high electroconductivity, especially, copper, is widely used as the material of the electric conductor. Further, a glass substrate is used as a substrate where the electric conductor is provided.
Since copper is unlikely to adhere to glass, a bonding layer is provided between the copper electrode and the glass substrate to improve bonding reliability. Chrome is widely used as the bonding layer, which should have an excellent affinity for both of copper and glass.
A guide is provided to the encoder head so that the encoder head slides relative to the encoder scale.
The guide includes a rotatable roller, and a surface of the glass substrate defines a belt-shaped guide surface where glass is exposed without being covered with the bonding layer. When the roller rolls on the guide surface, the encoder head smoothly slides in a longitudinal direction of the encoder scale at a constant interval relative to the encoder scale.
However, the rotation of the roller on the guide surface may cause electrostatic charge of the glass substrate. Electrostatic charge of the glass substrate, which causes electrical discharge and/or noise, is unfavorable for the encoder head. Further, electrostatic charge of the glass substrate often makes dust stick to the encoder scale, causing problems such as detection failure. In order to prevent electrostatic charge of the glass substrate causing the above problems, the glass substrate of a typical encoder scale is provided with an antistatic electrode.
FIGS. 5A to 5D and FIGS. 6A to 6D show an example of processes of manufacturing a typical encoder scale.
For manufacturing an encoder scale 104, a bonding layer 142 in the form of a film is first formed on a glass substrate 141, and then an electrode layer 143 is formed on the bonding layer 142. Subsequently, a resist 144 is applied to the electrode layer 143, and a predetermined mask pattern is formed by, for instance, photolithography.
FIG. 5A shows the state of the workpiece where the above processes have been completed.
Next, as shown in FIG. 5B, the electrode layer 143 is partially removed by etching using the resist 144 as a mask, thereby forming an electric conductor 143A and a copper mask 143B.
Next, as shown in FIG. 5C, the bonding layer 142 is partially removed by etching using the electric conductor 143A as a mask. A bonding body 142A and an antistatic electrode 142B for the electric conductor 143A are thus formed. The bonding body 142A has a shape identical to that of the electric conductor 143A in a plan view of the substrate 141 so that the electric conductor 143A can serve as a mask. However, the resist 144 still adheres to the workpiece in the above state, so that the resist 144 is removed as shown in FIG. 5D.
Next, as shown in FIG. 6A, the electric conductor 143A and the bonding body 142A thereunder are masked with another resist 147, and then the copper mask 143B is removed by etching. As shown in FIG. 6B, the antistatic electrode 142B is thus exposed.
After the resist 147 is removed as shown in FIG. 6C, the electric conductor 143A and the bonding body 142A thereunder are covered with a protection film 145A, which may be made of an insulating resin, and a grounding conductor 146 is connected to the exposed antistatic electrode 142B as shown in FIG. 6D. The encoder scale 104 is thus manufactured.
According to a method of manufacturing the typical encoder scale 104, a corrosive for etching the copper electric conductor 143A is different from a corrosive for etching the chrome bonding layer 142, so that the etching process shown in FIG. 5B and the etching process shown in FIG. 5C should be independent of each other.
Further, the additional processes shown in FIGS. 6A and 6B are necessary to provide the antistatic electrode 142B for grounding on the substrate 141. A lot of processes are thus necessary for manufacturing the typical encoder scale 104.